This foldr operator repeatedly applies the callable fn to a sequence
of elements from last to first. The elements are made of the tensors
unpacked from elems. The callable fn takes two tensors as arguments.
The first argument is the accumulated value computed from the preceding
invocation of fn. If initializer is None, elems must contain at least
one element, and its first element is used as the initializer.

Suppose that elems is unpacked into values, a list of tensors. The shape
of the result tensor is fn(initializer, values[0]).shape.

This method also allows multi-arity elems and output of fn. If elems
is a (possibly nested) list or tuple of tensors, then each of these tensors
must have a matching first (unpack) dimension. The signature of fn may
match the structure of elems. That is, if elems is
(t1, [t2, t3, [t4, t5]]), then an appropriate signature for fn is:
fn = lambda (t1, [t2, t3, [t4, t5]]):.

Args:

fn: The callable to be performed.

elems: A tensor or (possibly nested) sequence of tensors, each of which
will be unpacked along their first dimension. The nested sequence
of the resulting slices will be the first argument to fn.

initializer: (optional) A tensor or (possibly nested) sequence of tensors,
as the initial value for the accumulator.

parallel_iterations: (optional) The number of iterations allowed to run
in parallel.

back_prop: (optional) True enables support for back propagation.

swap_memory: (optional) True enables GPU-CPU memory swapping.

name: (optional) Name prefix for the returned tensors.

Returns:

A tensor or (possibly nested) sequence of tensors, resulting from applying
fn consecutively to the list of tensors unpacked from elems, from last
to first.